A group communications system can be a special communications system with a scheduling capability. The special communications system can provide a unidirectional call capability, and allow a user equipment (UE) to communicate with one or more other UEs. As increasingly rich mobile services and Long Term Evolution (LTE) technologies develop, it may be possible to implement rich group services in a common LTE network. In LTE group communication, data may be transmitted in a unicast manner and a multicast manner. In the multicast transmission manner of the group communication, data is transmitted by using an existing multimedia broadcast/multicast service (MBMS) technology.
In an LTE group communications system architecture, a group communication service application server (GCS AS) may be disposed on a network side. The GCS AS can be a third-party application server, and can be configured to implement functions such as group member management and MBMS bearer activation initiation in a group communication process. Generally, the GCS AS transmits data based on the Real-Time Transport Protocol (RTP). When the GCS AS needs to send data, the GCS AS may transmit the data in the unicast manner and the multicast manner.
FIG. 1 is a schematic diagram of structural composition of a user plane in a group communications system. If the GCS AS sends an RTP packet in the unicast manner, the GCS AS sends the RTP packet to UE by using a packet data network (PDN), and the UE receives the packet from the GCS AS. If the GCS AS sends an RTP packet in the multicast manner, the GCS AS first sends the RTP packet to a broadcast/multicast service center (BM-SC). After receiving the RTP packet, the BM-SC does not modify the RTP packet, but only binds an IP data flow from the GCS AS to a corresponding MBMS bearer, and then sends the RTP packet from the GCS AS to UE by using an MBMS gateway (MBMS GW) and an EnodeB or Evolved Node B (eNB). The UE receives the packet from the BM-SC. An RTP packet transmission scheduling period is 20 ms, and a unicast transmission delay is 40 ms, that is, a delay between a time at which the GCS AS starts to send the first RTP packet and a time at which the UE receives, by means of unicast, the first RTP packet sent by the GCS AS is 40 ms. A multicast transmission delay is 160 ms.
As a UE location changes, a manner of transmitting an RTP packet between the GCS AS and the UE also changes. As shown in FIG. 2, in a moving process, the UE may move from a unicast coverage to an MBMS coverage, and the manner of transmitting an RTP packet between the GCS AS and the UE is switched from the unicast manner to the multicast manner. Conversely, when the UE moves from the MBMS coverage to the unicast coverage, the manner of transmitting an RTP packet between the GCS AS and the UE is switched from the multicast manner to the unicast manner.
The unicast manner is different from the multicast manner in terms of an RTP packet transmission delay. As such, RTP packets received by the UE on different data transmission paths that are used before and after switching may become out of order and out of synchronization. For example, a packet may be lost or repeatedly received.